The present invention relates generally to suspension systems for motor vehicles and, more particularly, to a suspension system in which damping is actively controlled via switched reluctance motors and which defines a fail-safe damping for the suspension system in the event of failure within the system.
When a motor vehicle is driven, springs in its suspension system compress and expand to absorb shocks which would otherwise be transmitted to occupants of the vehicle. Once deflected, the springs continue to oscillate until they eventually return to their original state. Since spring oscillations create handling problems and reduce ride comfort of the vehicle, shock absorbers are used to dampen the oscillations of the springs and thereby stabilize the suspension system and return the springs to their original state substantially more quickly.
Traditionally, shock absorbers provide damping by means of hydraulic systems wherein hydraulic fluid, pistons, valves and the like define the damping provided by the shock absorber. More recently, shock absorbers have been constructed to incorporate electric motors to perform the damping operations. Both linear and rotary motors have been fashioned to operate suspension elements or shock absorbers within a motor vehicle suspension system. For rotary motors, the linear motion between sprung and unsprung masses of the chassis and wheels, respectively, of a motor vehicle are converted into rotational motion and controlled through the motor. A variety of motors including permanent magnet and switched reluctance have been incorporated in shock absorbing suspension designs.
In any damping suspension system, there is a concern for what damping rates will be applied in the event of failure within the suspension system. Fail-safe operation of a damping suspension system incorporating permanent magnet motors can be accomplished by connecting the motor windings across a load as disclosed in U.S. Pat. No. 5,070,284 which issued Dec. 3, 1991 and is assigned to the assignee of the present application. However, switched reluctance motors, as opposed to permanent magnet motors, have no inherent means of self-excitation and, as such, are incapable of developing torque in the absence of current flow in their windings.
U.S. Pat. No. 5,028,073 relates to a dynamic vehicle suspension system having shock absorbers incorporating rotary motors which are identified as permanent magnet and switched reluctance motors. Power is supplied to the motors from a power supply or a capacitor filter bank during normal operation of the suspension system. In the event of system failure, the windings of the machine are short-circuited. Unfortunately, as noted above, a switched reluctance motor requires an external source of excitation current such that short-circuiting the windings in a switched reluctance motor would result in loss of substantially all damping in the suspension system.
Thus, while switched reluctance motors can be used in suspension systems of motor vehicles and are desirable because of their simple construction and corresponding low costs, a problem can result in the event of failure within such a system. It is apparent that a need exists for an improved control system for operation of switched reluctance motors for use in motor vehicle suspension systems to provide a satisfactory fail-safe damping rate for the suspension system in the event of a failure within the system.